The Abbey Newsletter

Volume 26, Number 6
Oct 2003

Chlorine Dioxide: A Treatment for Mold in Libraries

By Kristina L. Southwell

Mold has been recognized as a serious threat to the safety and longevity of library materials for many years. Its ability to weaken, discolor, and destroy books and other paper items is well known. Although most librarians and archivists are aware of mold's negative effects on collections, many do not have first hand experience in dealing with an active mold bloom until mold is unexpectedly discovered in their library's stacks. In 1991, employees at the University of Oklahoma Libraries received a quick course in mold abatement, when mold was found growing in a book stack area.1

The mold bloom, initiated by a combination of roof leaks and malfunctioning air handling systems, occurred in the oldest portion of the building. The book stacks in this area house older, Dewey-classified books on steel shelving. Low ceilings, narrow aisles and heavy doors that close off the decks from the other more frequently used areas of the library create an environment that often has less than adequate airflow. The lack of good air circulation combined with the accidental added moisture and heat produced a climate well-suited for mold growth.

As with any situation in which a mold bloom occurs, attention was first given to stabilizing the temperature and humidity by repairing the roof leaks and the air handling system. When these factors were brought under control, library workers began using a thymol solution to wipe mold from the books. When concerns arose about possible carcinogenic effects of thymol. The treatment was halted and a microbiologist was consulted for advice on an alternative chemical treatment. Library administrators chose chlorine dioxide for its effectiveness as a sporicide, and for its safety level for library employees and patrons.

Chlorine dioxide is commonly used as a biocidal agent in water treatment applications, paper manufacturing, and many branches of the food processing industry.2 It can be used in both aqueous and gaseous forms, and has recently gained notoriety as the agent used to combat anthrax in the Hart Senate Building on Capital Hill, and U.S. postal facilities in New Jersey and Washington, D.C. Because of its history of use by various industries, statistical data on chlorine dioxide's safety is available from the U.S. Environmental Protection Agency, and the U.S. Occupational Safety and Health Agency.3

Several methods of using ClO2 were tried in the next few years, but finally (for a particularly hard-to-treat area in a closed collection), self-activating packets of chlorine dioxide, marketed under the name Aseptrol."4

The small packets contain powder that reacts with humidity in the air to release chlorine dioxide in gaseous form. A total of six packets were hung with wire ties between the bookshelves, in a closed area that measures about 12,000 square feet. This approach stopped the mold bloom, and no subsequent infestations have been found in this area.

As a result of the success of chlorine dioxide in these areas, the University Libraries continue to use chlorine dioxide packets to control mold growth in emergency situations, as well as areas that require regular treatment due to fluctuating temperature and humidity levels. In the spring of 2003, a steam pipe burst under the branch library that houses architecture materials, creating an ideal hot and humid environment for a mold bloom. Chlorine dioxide packets were immediately hung after the steam pipe was repaired, and to date no mold growth has been detected.

The University Libraries also have found that chlorine dioxide packets are particularly well suited for controlling mold growth in special collections areas, which often consist of small, enclosed storage areas that do not receive much traffic from library employees or patrons. The self-activating chlorine dioxide packets can be hung and left to work without requiring constant monitoring by library staff. The regular presence of mold eradication chemicals in areas with unstable environments or a past history of mold infestation provides an added level of security for susceptible collections.

As a follow-up study, in 2000 the University Libraries conducted a test on the library materials that were treated with chlorine dioxide in the 1991 mold outbreak in the decks. This test was designed to determine whether any unintended negative effects resulted from the books' exposure to chlorine dioxide. The tests focused specifically on paper acidity by measuring the pH levels of the paper in books that had been wiped and fogged with chlorine dioxide, and compared them to similar books that had no exposure to chlorine dioxide. The tests were conducted using a microcomputer pH meter with a flat sensor electrode attachment. The meter allowed for direct measurement of paper pH, expressed in numeric values, and had an added temperature electrode that compensated for fluctuations in temperature during the testing process. A group of 250 books that had been exposed to chlorine dioxide were compared to a similar group that received no chlorine dioxide treatment. The results were satisfactory, showing that the treated group's average pH registered at 5.22, and the untreated group's average pH was 5.31. An independent means test was performed on the pH statistics, which showed that no significant difference in pH levels (T-value=0.943) existed between the paper of treated and untreated books.

Many libraries with mold problems may find that proper temperature and humidity controls and non-chemical techniques (such as book vacuuming with HEPA-filtered machines) work just fine to control a moderate outbreak. However, those who find that standard treatments are not enough to stop mold growth can consider chlorine dioxide as an alternate solution.

[Recently, OU Libraries have come to suspect that the level of the active chemical, ClO2, has been lessened in their most recent shipments of the packets. Individual results using this method could therefore vary. Abbey Publications will continue to stay up-to-date on chlorine dioxide research. -Ed.]


1. Pat Weaver-Meyers, Wilbur A. Stolt, and Barbara Kowaleski's "Controlling Mold on Library Materials with Chlorine Dioxide: An Eight-Year Case Study," The Journal of Academic Librarianship 24 (1998): 455-458.

2. Simpson, G.D., R.F. Miller, G.D. Laxton, W.R. Clements. "A Focus on Chlorine Dioxide: The 'Ideal' Biocide." Online, Chlorine Dioxide Water Treatment Resource Center. Available: (accessed May 12, 2003).

3. Toxicological Review of Chlorine Dioxide and Chlorite, U.S. Environmental Protection Agency (Washington, D.C.). Available: (accessed May 12, 2003): the most current statistics from OSHA are available at (accessed May 12, 2003).

4. These packets can be obtained from most boating supply stores, which refer to them as "mildew control bags" for use on boats and boat houses.

Kristina L. Southwell is Assistant Professor of Bibliography/Manuscripts Librarian at the University of Oklahoma Libraries, Western History Collections. She can be contacted at
Reprinted with permission from Archival Products News 10 (3) (

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